This disclosure relates to methods and devices useful in separating the strongest of a plurality of linearly combined signals and suppressing the strongest frequency. Since the methods and devices may operate under almost arbitrary conditions of both amplitude and exponent modulation on the signal to be suppressed, the methods and devices are particularly adapted to defeat electronic jamming. In addition the methods and techniques may be employed in the on-line measurement of relative powers of a desired signal and undesired signal or noise powers.
A number of effective related techniques for signal suppression have been described in applicant's U.S. Pat. No. 3,911,366. Those prior techniques, while proven effective against signals of constant amplitude but carrying exponent modulation, are generally either degraded by significant levels of amplitude modulation (as in the case of the so-called "feedforward techniques") or (as in the case of the so-called "dynamic trapping techniques") retain the sideband power caused by amplitude modulation at frequencies that exceed the width of the trapping frequency null. Basically, the dynamic trapping techniques subtract out the exponent modulation of the signal to be suppressed in order to collapse the width of the signal spectrum down to one line, which is then reduced or zeroed out by the trapping "notch".
The techniques disclosed herein provide a dynamic trapping technique that may be used to cancel out both amplitude modulation and exponent modulation on the signal to be suppressed. This has the effect of collapsing all sideband power, whether caused by amplitude or exponent modulation or a combination thereof, down to one spectral line to be nulled out.
Accordingly, it is an object of the present invention to provide a dynamic trapping technique capable of suppressing one of a plurality of signals wherein the signal to be suppressed is amplitude modulated.
It is another object of the present invention to provide a method of separating and suppressing one of a plurality of signals, wherein the suppressed signal is both amplitude modulated and exponent modulated.
It is yet another object of the present invention to provide an effective method for suppressing the power in the sidebands of a signal carrying both amplitude modulation and exponent modulation, in the presence of other signals.
One prior art method for suppressing amplitude modulation on a signal is to employ automatic gain control. This is usually accomplished by first detecting the envelope of the signal. If the AM contains very high frequency components, such as for example in abruptly stepped amplitude changes, the envelope detector must be capable of following the fast fluctuations or the abrupt amplitude steps very closely. Otherwise, so-called "diagonal clipping" will occur. This often imposes severe reaction-time (or time-constant) requirements on the design of the envelope detector circuit.
Accordingly, it is an object of the present invention to provide a simple technique for detecting the envelope of a signal.
It is another object of the present invention to provide a technique for envelope detection that is substantially free of the possibility of diagonal clipping.
It is yet another object of this invention to provide an alternative to using automatic gain control to effect the suppression of AM on a given signal.
The measurement of signal-to-noise ratio requires separation of a desired signal from a noise or interfering signal.
Accordingly, it is a further object of this invention to provide methods and devices for separating out a signal carrying both amplitude modulation and exponent modulation to facilitate the measurement of the relative powers or the power ratios of two or more combined signals.
Envelope detection is frequently employed in the processing of signals imposed on a carrier wave. Further, it has been determined that a signal proportional to a reciprocal of the envelope (or amplitude) of an amplitude modulated signal is useful for signal processing, particularly where the amplitude modulated signal is to be separated from other linearly combined signals.
Accordingly, it is yet another object of the present invention to provide a technique for deriving the reciprocal of the envelope of a signal carrying amplitude modulation.